1. Field of the Invention
The invention relates to hybrid electric vehicles having a traction motor, an internal combustion engine and a traction battery that can be charged using an external electrical grid.
2. State of the Art Discussion
A known hybrid electric vehicle powertrain may have an internal combustion engine, an engine-driven generator and an electric motor wherein the energy source for the engine is a hydrocarbon-based fuel and the energy source for the electric motor is a high voltage battery and generator sub-system. The battery may be charged by the engine using fuel stored in the vehicle or from an external electric grid. In the case of a hybrid electric vehicle in which the battery is charged from an external electrical grid, an all-electric vehicle drive range is possible while the engine is off, thereby avoiding production of undesirable engine exhaust gas emissions. This feature is of particular importance for vehicles operating in an urban environment. Such vehicles, which sometimes are referred to as “plug-in” hybrid electric vehicles, are capable also of achieving improved overall fuel economy compared to the fuel economy of comparable non-hybrid vehicles.
If the range of a given vehicle driving event is limited, such vehicles can be operated with the engine off altogether. The battery, which may be depleted a moderate amount to a state-of-charge state less than a calibrated maximum charge but greater than a minimum state-of-charge, can be recharged using the electrical grid during driving off-time.
There are three general classifications for hybrid electric vehicle powertrains; i.e., series hybrid electric powertrains, parallel hybrid electric powertrains and series-parallel hybrid electric powertrains, the latter including so-called power-split hybrid electric powertrain systems. In the case of a series hybrid electric vehicle powertrain, an internal combustion engine drives a generator, which converts mechanical engine power to electrical power. A portion of the electrical power is used to drive an electric motor, which converts electrical power back to mechanical power to drive vehicle traction wheels. The power not needed by the motor is used to charge a battery.
In the case of series-parallel and parallel gasoline-electric hybrid vehicles, mechanical engine power can be delivered to the traction wheels, and electric power can be delivered from a battery to a motor, which converts the electric power to mechanical power to drive the traction wheels. Power flow from the engine to the generator will occur when the battery is being charged. Transmission gearing forms parallel power flow paths to vehicle traction wheels.
A common misconception about series-parallel and parallel hybrid electric vehicle powertrains is that vehicle propulsion using only electric power improves overall fuel economy because the vehicle uses no hydrocarbon fuel when the engine is off. However, this is not the case because losses incurred by the electric motor and the battery during discharging and subsequent battery charging will degrade overall engine fuel economy as electric power is converted to mechanical power and mechanical power is converted to electric power. Therefore, in known parallel and series-parallel hybrid electric vehicle powertrains that use a hydrocarbon-based engine and an electric motor powered by a battery, the guiding design principle for energy management software strategy is to provide as much propulsion as possible with an internal combustion engine, while selectively using the electrical system to increase average overall operating efficiency of the engine. An example of selective engine use includes engine-off electric driving during low driver demand situations, or slight battery discharging or charging to adjust engine power to achieve maximum engine thermal efficiency.
A parallel or series-parallel hybrid electric vehicle powertrain that relies upon an external powertrain grid for charging the traction battery (i.e., a plug-in vehicle) can achieve the best fuel economy using electric-only propulsion because there is an external energy source available rather than gasoline stored in the vehicle. A new energy management software strategy, therefore, is required to realize the fuel economy improvements that can be gained using a plug-in series-parallel or parallel hybrid electric vehicle powertrain.